The present invention relates to an image forming apparatus for forming an image onto a transfer material, and particularly relates to an image forming cartridge for use in an image forming apparatus of the electrophotographic type such as a copying machine, a laser beam printer, or the like.
A copying machine and a laser beam printer are conventionally known types of electrophotographic image forming apparatus. Here, the laser beam printer will be described.
FIG. 3 is a diagram for schematically explaining the configuration of a laser beam printer. A laser beam printer 11 has a laser scanner 12. The laser scanner 12 is provided with a semiconductor laser 13 for outputting a laser beam modulated on the basis of an image signal. The laser beam emitted from the semiconductor laser 13 is incident into a polygon mirror 14 and deflected in accordance with the rotation of the polygon mirror 14. After passing through an f.theta. lens 15, the deflected laser beam is changed in its traveling direction through mirrors 16 and 17 and outputted from the laser scanner 12.
A photoreceptor drum 19 is disposed below the laser scanner 12 so as to rotate at a fixed speed. The laser beam outputted from the laser scanner 12 repeatedly scans a predetermined exposure position 21 of the photoreceptor drum 19 in its axial direction, that is, in the main scanning direction. A charge corotron 22 is provided in a position opposing the photoreceptor drum 19 slightly circumferentially spaced from the exposure position 21 so that a given point on the surface of the photoreceptor drum 19 is uniformly charged before it reaches the exposure position 21. The charged photoreceptor drum 19 is irradiated with the laser beam so that an electrostatic latent image corresponding to image information is formed on the drum surface. The electrostatic latent image is developed by a developing device 24 at a portion of the drum surface downstream along the path of rotation of the drum 19 from the exposure position 21. Members such as a developing roll 25 for developing an electrostatic latent image by means of magnetically spiked toner, a toner supply mechanism 26 for supplying the developing roll 25 with toner out of a cartridge, and the like, are provided in the developing device 24. A predetermined development bias voltage is applied to the developing roll 25.
The toner image formed on the photoreceptor drum 19 by the developing device 24 is moved to a position opposite to a transfer corotron 28 as the photoreceptor drum 19 rotates, and is electrostatically transferred onto recording paper (ordinary paper). Each of the charge and transfer corotrons 22 and 28 used in this example has such a configuration that a single corotron wire is stretched in an air space covered with a shielding member and a voltage application terminal is provided at one end of the wire.
Next, a recording paper feeding path will be briefly described. Sheets of recording paper (not shown) are stacked in a cassette tray 31 removably disposed in a lower portion of the laser beam printer 11. The uppermost sheet of recording paper in the cassette tray 31 is fed out of the tray 31 by means of a semicircular roll 32. In place of the semicircular roll 32, sometimes, another means such as a retard roll or the like may be used.
The fed out recording paper is advanced by feeding rolls 33 along a path shown by a broken line and stopped from advancing once when the paper has reached front ends of resist rolls 34. Thereafter, an electromagnetic clutch (not shown) makes the resist rolls 34 start to rotate in synchronism with the rotational position of the photoreceptor drum 19 so that the stable feeding of the recording paper at a fixed speed is started. Thus, the recording paper passes in a timed manner between the photoreceptor drum 19 and the transfer corotron 28. The transfer corotron 28 performs discharging only at the time of this passage of the recording paper so that the toner image on the photoreceptor drum 19 is electrostatically attracted toward the transfer corotron 28 and transferred onto the recording paper. The back surface of the recording paper, after toner image transfer, is erased or discharged by means of erasure needles (not shown) arranged downstream from the transfer corotron 28 so that the recording paper is separated from the drum surface during erasive. After being fed along a feeding path of a predetermined length so as to release its tension, the recording paper, now separated from the drum surface, is sent to a fusing device made of a pair of rolls, that is, a heat roll 6 and a pressure roll 8. In the fusing device, the recording paper passes between the heat and pressure rolls 6 and 8 which are nip rolls separated by a predetermined width. At this time, the recording paper surface carrying the toner image transferred thereto faces the heat roll 6 while the recording paper is pressed by the pressure roll 8 against the heat roll 6 allowing for efficient heat conduction. The heat roll 6 is controlled to be at a fixed high temperature. In this state, the toner image on the recording paper is thermally fixed on the paper surface.
A selector valve 38 is provided in the outlet of the fusing device so as to switch the path for feeding the recording paper after fusing. By the switching operation of the selector valve 38, the recording paper after fusing travels straight so as to be discharged in the first discharge direction 39, or the paper after fusing is turned rightward (as shown in FIG. 3) so as to be discharged from an upper portion of the laser beam printer 11 in the second discharge direction 41 substantially opposite to the first discharge direction 39. The reason for the two discharge directions as described above is to make it possible to select whether the recording paper is to be discharged with its recording surface turned upward or downward. If recording paper is discharged with its recording surface turned downward by selecting the second discharge direction 41, sheets of recording paper successively printed page by page can be bound by a stapler as they are in proper numerical order.
The toner image which has not been transferred onto the recording paper is removed from the drum surface by a cleaning device 43 arranged downstream along the direction of rotation of the drum 19 from of the transfer corotron 28. The cleaning device 43 is provided with a blade 44 for scraping toner from the drum surface and a rotary body 45 for displacing toner particles accumulated under the blade 44 to a storing position in the rear side of the cleaning device 43.
Then, the photoreceptor drum 19 cleaned by the o cleaning device 43 is discharged by means of a discharge device 62. Generally, an erasure lamp is used as the discharge device 62, and an LED is used as the lamp. Further, the discharge operation is sometimes performed before cleaning depending on the kind of the apparatus. Then, the photoreceptor drum 19 discharged by the discharge device 62 is charged again by the charge corotron 22, and the operation is shifted to the next cycle.
Recently, miniaturization of image forming apparatuses such as laser beam printers or the like has been progressed. Miniaturization of photoreceptor drums has been also progressed with the miniaturization of the image forming apparatus. At present, photoreceptor drums have been miniaturized to the diameter of about 30 mm from about 80 mm several years before.
The miniaturization of a photoreceptor drum causes a problem concerning space available for constituent parts required to be arranged around the photoreceptor drum. As seen in the above explanation about the outline of the laser beam printer, it is necessary to provide a charger, an exposure device, a developing device, a transfer device, a separation device, a cleaning device, and a discharge device around the photoreceptor drum. Of those constituent parts, the parts other than the exposure device require a relatively large space, while only the exposure device occupies a very small space if a laser scanner is used. Particularly, it is necessary to prepare a considerably large space for the transfer and separation devices because it is necessary to secure a space for making a transfer material (recording paper) pass between the photoreceptor drum and each of the transfer and separation devices.
It is therefore very difficult to design the remaining parts, that is, the charger, the developing device, the cleaning device, and the discharge device. The discharge device, which has a tendency to be considered lower in importance than the charger, the developing device, and the cleaning device, is sometimes displaced to a position immediately after the separation device and before the cleaning device. If the discharge device is arranged in such a position, however, discharge operation is performed over non-transferred toner on the photoreceptor drum, so that the printing quality is deteriorated because the surface of the photoreceptor drum on which toner remains cannot be sufficiently discharged. It is therefore desirable to perform discharge operation immediately after cleaning.
An image forming cartridge called an EP (electronic printing) cartridge, in which a photoreceptor drum is integrally provided with a charger, a developing device, and a cleaning device, and which is removably attached to the body of an image forming apparatus, has become popular because of its superiority in maintenance and reliability. In the case of such an EP cartridge, a shutter is sometimes attached so as to prevent an external damage or light fatigue of the photoreceptor drum from occurring when the EP cartridge is taken out of the body of the image forming apparatus because it is necessary to keep portions of the photoreceptor drum uncovered adjacent the transfer and separation devices.
With respect to exposure, in the case of a laser beam printer, it is necessary to provide a slit-shaped light path having a width of several few mm between, a charger and a developing device. With respect to discharge, it is necessary to provide a light path having a width of at least about 10 mm so as to irradiate a photoreceptor drum with light from an LED. To attach the LED in the EP cartridge causes a problem in cost, and the device is generally configured so that a light path having the foregoing width is prepared in the EP cartridge, the LED being attached to the body of the apparatus. As disclosed in Japanese Patent Unexamined Publication No. Sho. 60-63549, a cover is attached to such a light path so as to prevent dust or the like from entering or to prevent light fatigue of the photoreceptor from occurring.
Thus, it is considerably difficult to achieve miniaturization while maintaining required printing quality. Particularly, it is difficult to provide a secure discharge light path. Further, since reliability is reduced as the number of movable members increases, as by the addition of a cover, it is desirable to reduce the number of movable members as much as possible.